1. Field of the Invention
The present invention relates generally to the field of lens systems, and specifically, to a compact zoom lens system that has a diffractive surface and an anti-aliasing feature.
2. Background Information
Prior art zoom lens systems operating in the f/2.8 relative aperture domain and covering fields of view approaching 50 degrees require many expensive lens elements to achieve usable image quality. Usually, these elements are made of optical glasses that have very different dispersion properties to permit good correction of chromatic aberrations. Moreover, zoom lens systems requiring many elements must be several times their longest focal length when measured from their front vertex to an image surface. A compact form that operates in the stated relative aperture and field angle domain requires high-index optical glasses which makes the lens system very expensive to manufacture.
Moreover, optical systems that discretely sample images on detector arrays produce aliasing effects in the displayed image. Color Filter Arrays ("CFAs") integrated into discretely sampled detector arrays introduce color-aliasing effects. An example of aliasing effects in a monochrome (black and white) CCTV image are Moire effects that cause an object to move and change patterns when the object moves relative to a charge coupled device ("CCD") detector array and has dimensions commensurate with the pixel dimensions of the detector array. An example of color aliasing effects in a CFA CCD detector system is the similar Moire effects that cause an object to move, change patterns, and change color when the object moves relative to the detector array and the object has dimensions commensurate with the pixel dimensions of the detector. For example, when a building, having very narrow columns, is carefully observed on a TV system using a CFA detector system in the camera, some columns will have a red tint, some a green tint, and some a blue tint. As the camera moves, the tinted pattern shifts from one column to another.
Recently, anti-aliasing features have been introduced as molded structures near the aperture stop of a lens system. These newer-art devices typically use a single, very weak (i.e., having a height of a few wavelengths of light), four-sided pyramid to form four displaced images at the detector surface. Because each of these four images are formed by a section of the aperture that looks like one-quarter of a pie, the image structure is a function of the size of the aperture stop, the object distance, and the zoom position. In essence, this pyramidal feature divides the incident wavefront from a point object into four pie-shaped wavefronts that converge to four blurred images at the detector. These four tilted wavefront sections change separation as a function of object distance and zoom position. However, this type of anti-aliasing feature is undesirable because the amount of anti-aliasing changes as the size of the aperture stop, the object distance, and the zoom position changes.
Other prior art lens systems have attempted to solve aliasing effects by providing anti-aliasing features that are molded onto a lens surface (using aspheric surface profiles) which introduces spherical aberration to blur the image of a point object. However, this design solution is very sensitive to the relative aperture and object distance and is not acceptable for the intended application.
Another problem with zoom lens systems is the suppression of stray light. Because the mechanical flanges of the lens elements are large and have non-optical surfaces, light entering the front of the flange can propagate through the flange and disperse across the detector array surface.
Accordingly, there is a need in the art for a method and apparatus for a compact zoom lens system that corrects chromatic aberrations over the entire range of the zoom lens system, minimizes aliasing effects, and suppresses stray light while maintaining a low cost.